The present invention relates, in general, to a corrosion inhibitor for concrete having steel reinforcing rods (rebar) therein and, more particularly, to an admixture which can be mixed with the concrete when in the plastic state and which prevents corrosion of the steel reinforcing rods over an extended period of time when the concrete is exposed to chloride ion environments.
Reinforced concrete structures, such as highways, bridges, parking garages, and the like are very susceptible to corrosion from common chloride deicing salts which are applied to their respective surfaces and which cause corrosion of the steel reinforcing rods (rebar) which are an integral part of their structure. Similarly, reinforced concrete structures which are exposed to aggressive marine environments, such as piers, docks and bridge supports, are also susceptible to corrosion of the reinforcing rods therein. In either case, such corrosion is usually caused by chloride ions that penetrate through the surface of the concrete and contact the reinforcing rods. The electrochemical process by which corrosion of the reinforcing rods and rod degradation occurs is well known.
Under highly alkaline conditions, such as that which exists in Portland cement concrete, an oxidized film forms on the steel reinforcing rods inhibiting the corrosion of the rods. (The steel rods are said to have become xe2x80x9cpassivated.xe2x80x9d) However, when chloride ions are allowed to penetrate into the concrete and reach the reinforcing rods, the first phase of the corrosion process (the initiation phase) commences. In this phase, there is no noticeable weakening of the concrete structure, but carbonation and chloride ion penetration occurs. Carbonation reduces the pH of the concrete, thus reducing the corrosion protection usually provided to the reinforcing rods by the alkaline concrete. Eventually, the passivity of the steel reinforcing rods breaks down as the oxidized film on the rods is broken and decays. Such breakage of the film generally occurs locally exposing the steel rods. As the oxidized film decays, the electrical resistance of the steel rods, i.e., the property that prevents the surface of the steel rods from polarizing and forming anodes and cathodes, is compromised. As a result, the small exposed portion of a steel rod acts as an anode, and the larger unexposed portion of the rod, still covered by the oxidized film, acts as a cathode resulting in the creation of a potential difference between the anode and the cathode. When the potential difference between the anode and the cathode is great enough, the steel reinforcing rod begins to corrode, i.e., metal ions are removed from the rod at its anode. As a result, corrosion takes place in spots (pitting) along the surface of the steel reinforcing rod resulting in the commencement of the second phase of the corrosion process (the propagation phase).
During the propagation phase, the effective sectional area of the steel reinforcing rod is progressively reduced by the corrosion causing a significant reduction in the strength of the rod. As the number of corrosion spots (pits) increases, they interconnect with one another spreading over the entire surface of the steel rod. In the initial stages of corrosion, ferrous hydroxide is formed which immediately oxidizes into iron oxides which are the main components of rust. In the course of the rust formation, the corroding rod expands at the point of rust formation. The localized expansion of the steel reinforcing rod caused by the formation of rust results in a high expansion pressure being applied to the concrete surrounding the expanded portion of the rod causing cracks to develop in the concrete along the surface of the rod. As the cracks develop in the concrete, additional chloride ions are permitted to contact the steel reinforcing rods, accelerating the corrosion of same and the spalling of the concrete surface. If corrosion and spalling are permitted to continue, the steel reinforcing rods, as well as the surrounding concrete, deteriorate to the point where the structural integrity of the concrete structure may be jeopardized. In order to remedy this condition, the removal and replacement of a substantially large area of concrete is required which is a very costly process.
Several approaches have been taken to repair concrete structures which have undergone or are susceptible to corrosion deterioration of the steel reinforcing rods therein. For example, severely deteriorated concrete can be removed and an overlay applied to the deteriorated structure. Large areas of chloride contaminated concrete, however, will remain in place, and although the corrosion and deterioration process will be slowed, the corrosion process continues. Alternatively, scarification of the top portion of concrete, e.g. on a bridge deck, can be utilized to remove a major portion of the chloride contaminated concrete permitting the application of a corrosion inhibiting agent to the concrete surrounding the steel reinforcing rods. After a corrosion inhibiting agent has been applied to the surrounding concrete, a new concrete overlay is formed thereon. A preferred rehabilitative technique requires complete removal of the concrete surrounding the steel reinforcing rods prior to the application of a new overlay.
Until now the corrosion inhibiting admixtures that have been developed for mixing with concrete when in the plastic state have been limited in their ability to delay the onset of corrosion in the steel reinforcing rods within the concrete, i.e., the initiation phase of corrosion, or to slow such corrosion after it has started, i.e., the propagation phase of corrosion. In view of the foregoing, it has become desirable to develop an admixture that can be mixed with the concrete when in the plastic state and which significantly delays the onset of corrosion in the steel reinforcing rods within the concrete and slows such corrosion after it has commenced even when the concrete is exposed to chloride ion environments. It is also desirable for such an admixture to protect the reinforcing rods in concrete that has partially or completely carbonated reducing the pH of the concrete and accelerating the onset of the protective oxide film deterioration in a chloride containing environment. Ideally, the admixture would also maintain and preferably increase the pH of the concrete.
The present invention solves the problems associated with the prior art approaches to minimizing corrosion of steel reinforcing rods in concrete as well as other problems by providing a unique corrosion inhibiting admixture comprising a combination of organic (on the basis of amine) and inorganic (on the basis of nitrite) fractions that provide a synergistic effect when present in a specific ratio. The range of the optimal amine:nitrite ratio (% by weight) is between 1.5 to 2.5. The admixture is introduced into concrete when in the plastic state by placing same in the mix water during the batching process or at the construction site. The admixture is thoroughly distributed throughout the concrete to provide substantially uniform levels of corrosion protection within the concrete. In an alternate embodiment of the present invention, a portion of the nitrite is replaced with lithium nitrite to minimize any undesirable alkali-silica reactions in the concrete. It has been found that the introduction of either embodiment of the aforementioned admixture into concrete when in the plastic state significantly delays the onset of corrosion of the steel reinforcing rods within the concrete and slows such corrosion after it has commenced even when the concrete is exposed to aggressive, salt-bearing environments. In addition, the admixture increases the pH of concrete which has carbonated.